You'd take a look to Mage_Catalog_Block_Product_List_Toolbar
Sorter urls, in a default installation, use attribute codes, you can check this by calling
Mage::getSingleton('catalog/config')->getAttributeUsedForSortByArray();
Check catalog/product/list/toolbar.phtml, so you can see how Magento gets available orders, and their urls
<div class="sort-by">
<label><?php echo $this->__('Sort By') ?></label>
<select onchange="setLocation(this.value)">
<?php foreach($this->getAvailableOrders() as $_key=>$_order): ?>
<option value="<?php echo $this->getOrderUrl($_key, 'asc') ?>"<?php if($this->isOrderCurrent($_key)): ?> selected="selected"<?php endif; ?>>
<?php echo $this->__($_order) ?>
</option>
<?php endforeach; ?>
</select>
<?php if($this->getCurrentDirection() == 'desc'): ?>
<a href="<?php echo $this->getOrderUrl(null, 'asc') ?>" title="<?php echo $this->__('Set Ascending Direction') ?>"><img src="<?php echo $this->getSkinUrl('images/i_desc_arrow.gif') ?>" alt="<?php echo $this->__('Set Ascending Direction') ?>" class="v-middle" /></a>
<?php else: ?>
<a href="<?php echo $this->getOrderUrl(null, 'desc') ?>" title="<?php echo $this->__('Set Descending Direction') ?>"><img src="<?php echo $this->getSkinUrl('images/i_asc_arrow.gif') ?>" alt="<?php echo $this->__('Set Descending Direction') ?>" class="v-middle" /></a>
<?php endif; ?>
</div>